The Labour Party is promising to cap class sizes at 30 pupils across all schools in England if it wins next week’s election.
The pledge is an extension of a commitment in its manifesto to limit classes to this size at all primary schools.
The party said it would fulfil the pledge by recruiting nearly 20,000 extra teachers over five years.
The Conservatives said English schools were rising up international rankings.
The party has pledged an extra £14bn over the next three years for schools in England.
Labour’s pledge to recruit nearly 20,000 teachers is similar to a promise made two weeks ago by the Liberal Democrats.
Lib Dem education spokeswoman Layla Moran accused the party of trying to “copy” them, but added that Labour had “no hope of meeting this target”.
She added that Labour would not be able to “square these promises” with leaving the EU if voters back the party’s Brexit deal in its planned referendum, due to “thousands of EU teachers coming to work in schools each year”.
School funding ‘slashed’
Labour said the recruitment would be funded from an extra £25bn in schools spending over the next three years. The party has also committed to ensuring all teachers have formal teaching qualifications within five years.
Shadow education secretary Angela Rayner said Labour’s pledges would “transform education standards in this country for every child”.
“We will invest in record per pupil funding, restore the pupil premium and close the gap in support for children with special educational needs and disabilities, to give every child the support they need,” she said.
She accused the Conservatives of having “slashed school funding for the first time in a generation, leaving pupils taught by unqualified teachers, crammed in to super-sized classes, and not receiving the support they need”.
There is a teacher shortage in England, with the latest official statistics showing a 15% shortfall in numbers beginning training for secondary schools.
But Labour’s promise to recruit more may not be easy to deliver.
In subjects such as physics, fewer than half the required graduates have begun to train.
Schools hire teachers, not the government, and it is harder to attract and keep teachers to work in poorer areas.
The party also says it would cap all class sizes at 30. At the moment, just over 12% of secondary pupils are taught in classes with between 31-35 pupils.
In 2009, when Labour was last in government, it was just over 10%.
While slightly smaller class sizes could appeal to parents, the international evidence suggests it may make less of a difference to results than the quality of teaching.
The National Association of Head Teachers said 47,000 secondary teachers and 8,000 primary teachers would be needed by 2024 to keep pace with an expected increase in pupil numbers.
Its general secretary Paul Whiteman said: “We need significantly more recruits than Labour are suggesting just to meet rising demand, never mind reduce current class sizes.”
“The new recruits we need will not magically appear, and nor will they stay if we don’t also address the stress and unnecessary workload that is widespread in the system.”
Schools minister Nick Gibb said that in government Labour would “would wreck the economy, leaving no money for public services”.
He added that figures this week from the OECD’s international school rankings showed English schools had risen up the league tables.
He added that schools in Wales, where schools are run by the Labour-led devolved government, were the lowest performing within the UK.
“Conservative education reforms are improving standards in our schools, meaning children can get a better start in life,” he added.
Labour are also announcing new plans to tackle homelessness, including £100m a year for emergency winter shelters, £600m to build new hostels, and £200m to refurbish existing ones.
In a speech later, shadow housing secretary John Healey will accuse the Conservatives of being “directly responsible” for an increase of people living on the streets.
A Conservative party spokesman said: “There is record investment going in to tackling homelessness – £1.2bn until April 2020 with a further £422m for 2020-21.”
The Conservatives, like Labour, have pledged to end rough sleeping within five years if elected to government.